Apparatus for fabrication of thin films

ABSTRACT

The invention relates to an apparatus for growing thin-films onto a substrate by exposing the substrate to alternate surface reactions of vapor-phase reactants for forming a thin-film onto the, substrate by means of said surface reactions. The apparatus comprises a vacuum vessel ( 1 ), a reaction chamber ( 2 ) with a reaction space into which the substrate can be transferred and which has infeed channels ( 6 ) for feeding therein the reactants used in said thin-film growth process, as well as outlet channels ( 4 ) for discharging gaseous reaction products and excess reactants&#39;. According to the invention, said reaction chamber comprises a base part ( 9, 10 ) mounted stationary in respect to the interior of said vacuum vessel ( 1 ) and a movable part ( 18 ) adapted to be sealably closable against said&#39;base part of said reaction chamber. The invention makes it possible to improve the cleanliness of the substrate load chamber and to reduce the degree of substrate contamination. The apparatus is intended for use in the fabrication of thin-films by means of the ALE method for semiconductor layer structures and display units.

REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. application Ser. No.09/769,562, filed Jan. 25, 2001, now U.S. Pat. No. 6,579,374, which is acontinuation of U.S. application Ser. No. 09/568,077, filed May 10,2000,now U.S. Pat. No. 6,562,140, which claims the foreign prioritybenefit under 35 U.S.C. 119 of Finish Application No. F1991078, filedMay 10, 1999.

FIELD OF THE INVENTION

The present invention relates to an apparatus according to the preambleof claim 1 for fabrication of thin-films.

BACKGROUND OF THE INVENTION

In an apparatus disclosed herein, a substrate placed in a reaction spaceis subjected to alternate surface reactions of at least two differentreactants suitable for fabricating a thin film. The vapor-phasereactants are fed in a repetitive and alternating manner each at a timefrom its own supply into a reaction space, wherein they are brought toreact with the surface of a substrate in order to produce a solid-statethin film product on the substrate. Reaction products not adhering tothe substrate and possible excess reactants are removed in gas phasefrom the reaction space.

Conventionally, thin-films are grown out using vacuum evaporationdeposition, Molecular Beam Epitaxy (MBE) and other similar vacuumdeposition techniques, different variants of Chemical Vapor Deposition(CVD)(including low-pressure and metallo-organic CVD and plasma-enhancedCVD) or, alternatively, the above-mentioned deposition process based onalternate surface reactions, known in the art as the Atomic LayerEpitaxy, shortly ALE, or Atomic Layer Deposition, (ALD). In thisdescription, the term “ALE” will be used. In the MBE and CVD processes,besides other variables, the thin film growth rate is also affected bythe concentrations of the starting material inflows. To achieve auniform surface smoothness of the thin films manufactured using thesemethods, the concentrations and reactivities of the starting materialsmust be kept equal on one side of the substrate. If the differentstarting materials are allowed to mix with each other prior to reachingthe substrate surface as is the case in the CVD method, the possibilityof mutual reactions between the reagents is always imminent. Hereinarises a risk of microparticle formation already in the infeed lines ofthe gaseous reactants. Such microparticles generally have adeteriorating effect on the quality of the deposited thin film. However,the occurrence of premature reactions in MBE and CVD reactors can beavoided, e.g., by heating the reactants not earlier than or only at thesubstrates. In addition to heating, the desired reaction can beinitiated with the help of, e.g., plasma or other similar activatingmeans.

In MBE and CVD processes, the growth rate of thin-films is primarilyadjusted by controlling the inflow rates of starting materials impingingon the substrate. By contrast, the thin film growth rate in the ALEprocess is controlled by the substrate surface properties, rather thanby the concentrations or other qualities of the starting materialinflows. In the ALE process, the only prerequisite is that the startingmaterial is provided in a sufficient concentration for film growth onthe substrate.

The ALE method is described, e.g., in Fl Patents Nos. 52,359 and 57,975as well as in U.S. Pat. Nos. 4,058,430 and 4,389,973. Also in FI PatentsNos. 97,730, 97,731 and 100,409 are disclosed some apparatusconstructions suited for implementing the method. Equipment for thinfilm deposition are further described in publications Material ScienceReport 4(7), 1989, p. 261, and Tyhjiotekniikka (title in English: VacuumTechniques), ISBN 951-794-422-5, pp.253-261.

In the ALE deposition method, atoms or molecules sweep over thesubstrates thus continuously impinging on their surface so that a fullysaturated molecular layer is formed thereon. According to theconventional techniques known from FI Patent Specification No. 57,975,the saturation step is followed by a protective gas pulse forming adiffusion barrier that sweeps away the excess starting material and thegaseous reaction products from the substrate. The successive pulses ofdifferent starting materials and the protective gas pulses formingdiffusion barriers that separate the successive starting materialspulses from each other accomplish the growth of the thin film at a ratecontrolled by the surface chemistry properties of the differentmaterials. To the function of the process it is irrelevant whether theyare the gases or the substrates that are kept in motion, but rather, itis imperative that the different starting materials of the successivereaction steps are separated from each other and arranged to impinge onthe substrate alternately.

Most vacuum evaporators operate on the so-called “single-shot”principle. Hereby, a vaporized atom or molecule can impinge on thesubstrate only once. If no reaction with the substrate surface occurs,the atom or molecule is rebound or re-vaporized so as to hit theapparatus walls or the vacuum pump undergoing condensation therein. Inhot-wall reactors, an atom or molecule impinging on the reactor wall orthe substrate may become re-vaporized and thus undergoing repeatedimpingements on the substrate surface. When applied to ALE reactors,this “multi-shot” principle can offer a number of benefits includingimproved efficiency of material consumption.

In practice, the “multi-shot” type ALE reactors are provided with areactor chamber structure comprised of a plurality of adjacently orsuperimposedly stacked modular elements of which at least some areidentical to each other and by milling, for instance, have reactionchambers made thereto with suitable cutouts and openings serving as theinlet and outlet channels. Alternatively, the substrates can be placedin an exposed manner in the interior of the vacuum vessel acting as thereaction space. In both arrangements, the reactor must be pressurized inconjunction with the substrate load/unload step.

In the fabrication of thin-film structures, it is conventional that thereactors are preferably run under constant process conditions stabilizedin respect to the process temperature, operating pressure as well as forother process parameters. The goal herein is to prevent the attack offoreign particles and chemical impurities from the environment on thesubstrates and to avoid thermal cycling of the reactors that is atime-consuming step and may deteriorate the process reliability. Inpractice, these problems are overcome by using a separate substrateload/transfer chamber. The substrate loading chamber communicates withthe reactors and is kept under a constant vacuum. The load and unloadsteps of the substrates are performed so that both the reactor and theloading chamber are taken to a vacuum, after which the valve (such as agate valve) separating the two from each other is opened, whereby arobotic arm constructed into the loading chamber removes a processedsubstrate from the reaction chamber and loads a new substrate.Subsequently, the valve is closed and the process may start after thesubstrate and the reactor have attained their nominal process values. Onthe other side, the processed substrate is transferred via anothercontrollable valve from the loading chamber to a vacuumized load lock,after which the load lock valve is closed. Next, the load lock may bepressurized, after which the substrate can be removed from the equipmentvia a third valve opening into the room space. Respectively, the nextsubstrate to be processed can be transferred via the loading chamberinto the reactor.

In conventional constructions, the substrate is placed on a heater sothat the robotic arm can move the substrate to a desired point in theinterior of the reactor, after which the substrate is elevated typicallywith the help of three pins directly upward for the duration of therobotic arm withdrawal. Next, the substrate is lowered onto a heatablesusceptor platform by lowering said pins below the surface level of saidsusceptor, whereby the substrate remains resting in a good thermalcontact with the susceptor.

In the above-described types of reactors, the gas flow enters thereaction space via a “shower head” located above the substrate so as todistribute the gas over the hot substrate, whereby the desired surfacereaction can take place and form a desired type of thin-film layer onthe substrate surface. If used in an ALE reactor, however, this type ofinfeed technique would require that, at the beginning and end of eachreactant infeed pulse, a period of a duration generally equal to that ofthe reactant pulse length would become indispensable in order to allowfor the homogenization of the gas concentration and flush-out of theprevious gas pulse. In practice, this would lead to the mixing of thereactant vapors with each other, whereby the ALE mode of film growthwould actually turn into a CVD process. By the same token, the processis hampered by a slow throughput, poor material utilization efficiencyand/or large thickness variations.

Furthermore, the walls of the vacuum vessel would respectively becomecovered with condensed layers of starting materials, and theconsequences particularly in conjunction with the use of solid-statesources would be the same as those discussed above.

The reason for running the ALE process in a batch mode is because theALE method is relatively slow as compared with many other types ofthin-film growth techniques. Batch processing, however is capable ofbringing the total growth time per substrate to a competitive level. Forthe same goal, also the substrate sizes have been made larger.

The ALE method also can be utilized for depositing composite layerstructures, whereby a single run can be employed for making a pluralityof different film structures in a single batch. Thus, also theprocessing time per fabricated unit can be reduced.

The large stack assemblies required in batch processes are typically puttogether in some auxiliary space, after which they must be transferredas compact units into the interior of the opened reactor. Typically, thebake-out heating of the reactor chamber structures takes a few hours(1-4 h) followed by the processing step (taking about 2-4 h to athickness of 300 nm Al₂O₃), and the cooling lasts up to tens of hoursdepending on the size of the reactor construction. Furthermore, acertain time must be counted for the dismantling and reassembly of thereactor chamber.

The proportion of the processing time to the work time required by theother processing steps becomes the more disadvantageous the thinner arethe thin films (e.g., in the range 1-50 nm) to be grown, whereby theduration of the actual deposition step may last from one to severalminutes only. Then, an overwhelming portion of the total processing timein respect to actual processing time is used for heating/cooling thereactor chamber structure, pressurization of the reactor,dismantling/reassembly of the reactor chamber, bringing the system tovacuum and reheating the same.

It is an object of the present invention to overcome the drawbacks ofthe prior-art technology and to provide an entirely novel apparatus forgrowing thin films of homogeneous quality using the ALE method in acommercial scale. It is a particular object of the invention to providean apparatus construction suitable for fabricating very thin films undersuch circumstances in which the reactor and the structure forming thereactor chamber are all the time kept under stabilized processconditions, whereby the heating, pressurizing and vacuum pumping cyclesare performed in respect to the substrates alone. It is a further objectto provide a reactor design that allows single wafer processing, usingthe advantages of the previous ALE reactors, viz. minimal reactionvolume, aerodynamic design for smooth passing of pulses without deadvolumes.

The goal of the invention is achieved by virtue of a novel concept inwhich the benefits of a reactor chamber structure and those of acold-wall ALE reactor are combined with those of reactor equipped withan internal loading chamber through designing reaction chamberconstruction such that can be opened and closed in the interior of thereactor for the substrate load/unload steps. The reactor volume is sosmall that there is not enough space for wafer handling. Therefore, thereaction chamber needs to be opened, not by a valve but by taking partsof the reactor apart, for wafer transfer. To prevent exposure of thereaction chamber to the ambient, a second chamber is built around thereaction chamber.

Accordingly, the reaction chamber construction according to theinvention comprises at least one part movable with respect to theremaining part of the reaction chamber and adapted to be sealablyclosable against said remaining part of the reaction chamber such that asubstrate can be loaded / unloaded into and out of the reaction chamber.According to one embodiment, which will be described in more detailbelow, the reactor chamber comprises at least two basic parts, namely astationary base part of the reactor structure and a movable part of saidstructure, the latter being adapted to be sealably closable against saidbase part, whereby the reaction space remaining between said base partand said movable part attains the characteristic shape required fromsuch a volume. One combination element formed by the base part and themovable part of the reaction chamber structure acts as a substratesupport platform, or base, on which a substrate can be positioned bymeans of a robotic arm or by the movements of a given part of said arm.However, the basic idea is to have reactor chamber parts which aremovable with respect to each other, e.g. one horizontally, the othervertically, or both vertically.

More specifically, the apparatus according to the invention isprincipally characterized by what is stated in the characterizing partof claim 1.

The invention offers significant benefits. Accordingly, the constructionaccording to the invention counteracts to the contamination of theintermediary spaces of the reactor, whereby also the cleanliness of theloading chamber is improved, the contamination risk of substratesreduced and the base pressure can be pumped down at a faster rate. Thecommercial processing of single substrates facilitated by the apparatusaccording to the invention offers the further benefit that eachsubstrate can be processed in a tailored manner if so desired. Moreover,the ALE process need not anymore be run in a batch mode if the otherproduction units are operated in a continuous mode.

The apparatus according to the invention is also applicable to thefabrication of different types of thin films including those of thesemiconductor variety, as well as thin-film structures of display unitsby means of the ALE method.

In the following, the invention will be elucidated with the help of adetailed description. A preferred embodiment of the invention isillustrated in the appended drawings in which

FIG. 1 shows a top view of an assembly diagram for a simplifiedconstruction of a reactor embodiment according to the invention:

FIG. 2 shows a side view of the a simplified construction of the reactorembodiment; and

FIG. 3 shows an enlarged sectional side view of the simplifiedconstruction of the reactor embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the context of the present invention, the term “reactant” refers to avaporizable material capable of reacting with the substrate surface. Thereactants may be solids, liquids or gases. In the ALE method, reactantsbelonging to two different groups are conventionally employed. The term“metallic reactants” is used of metallic compounds or even elementalmetals. Suitable metallic reactants are the halogenides of metalsincluding chlorides and bromides, for instance, and metalorganiccompounds such as the thd complex compounds. As examples of metallicreactants Zn, ZnCl₉, TiCl₄, Ca(thd)₂, (CH₃)₃Al and Cp₂,Mg may bementioned. The term “nonmetallic reactants” refers to compounds andelements capable of reacting with metallic compounds. The latter groupis typically represented by water, sulfur, hydrogen sulfide and ammonia.

In the present context, the term “protective gas” gas is used to referto a gas which is admitted into the reaction space and is capable ofpreventing undesired reactions related to the reactants and,correspondingly, the substrate. Such reactions include e.g. thereactions of the reactants and the substrate with possible impurities.The protective gas also serves to prevent reactions between substancesof different reactant groups in, e.g., the inflow lines. In the methodaccording to the invention, the protective gas is also advantageouslyused as the carrier gas of the vapor-phase pulses of the reactants.According to a preferred embodiment, in which reactants of differentreactant groups are admitted via separate inlet manifolds into thereaction space, the vapor-phase reactant pulse is admitted from oneinflow channel while the protective gas is admitted from another inflowchannel thus preventing the admitted reactant from entering an inflowchannel serving another reactant group. Of protective gases suited foruse in the method, reference is made to inert gases such as nitrogen gasand noble gases, e.g., argon. The protective gas may also be aninherently reactive gas such as hydrogen gas serving to preventundesirable reactions (e.g., oxidization reactions) from occurring onthe substrate surface.

According to the invention, the term “reaction space” includes both thatpart of the reaction chamber in which the substrate is located and inwhich the vapor-phase reactants are allowed to react with the substratein order to grow thin films as well as the gas inflow/outflow channelscommunicating immediately with the reaction chamber, said channelsserving for admitting the reactants into the reaction chamber (viainflow channels) or removing the gaseous reaction products of thethin-film growth process and excess reactants from the reaction chamber(via outflow channels). According to the construction of the embodiment,the number of the inflow and outflow channels, respectively, can bevaried from one upward. They may also be located at opposite ends of thesubstrates whereby the outflow orifice corresponding to each reactantgroup is located at the end of the inflow manifold of the other group,advantageously separated therefrom by means of a gas separating plate.The gases may also be fed onto the substrate alternately from oppositedirections. In this manner it is possible to compensate any observedstronger film growth at the inflow end of the substrate. Also theexhaust suction from the outflow channel in such an arrangement musttake place in an alternated manner.

The gas feed inlet openings can be arranged along a curved line, e.g.along a line having a radius of curvature somewhat greater than theradius of the circular substrate to be treated. By means of thisarrangement it is possible to obtain an even gas front long thesubstrate.

Herein, the term “substrate surface” is used to denote that top surfaceof the substrate onto which the vapor-phase reactant flowing into thereaction chamber primarily impinges. In practice, said surface, duringthe first cycle of the thin-film growing process is constituted by thesurface of the substrate such as glass or silicon wafer, for instance;during the second cycle the surface is constituted by the layer formedduring the first cycle and comprising the solid-state reaction productwhich is deposited by the reaction between the reactants and is adheredto the substrate, etc.

According to the invention, the reactor chamber is fitted within an air-and gas-tight containment, such as a vacuum vessel. Such a reactionchamber comprises e.g. at least two parts, at least one of which ismovable with respect to the remaining part of the reaction chamber andadapted to be sealably closable against said remaining part of thereaction chamber such that a substrate can be loaded and unloaded intoand out of the reaction chamber. The parts can be moved with respect toeach other (one or both); one horizontally (perpedicularly to thelateral plane of the substrate), the other vertically (laterally); orboth vertically (laterally). The parts of the reaction chamber arecontained in a second chamber, which preferably comprises a vacuumvessel, both during processing and during loading and unloading of asubstrate. Thus, there is arranged a double-containment of thesubstrate.

According to a preferred embodiment, the reaction chamber comprises twoparts, one of which being mounted stationary in respect to said vacuumvessel, while the other is designed movable. The transfer of the movablepart can take place laterally, that is, parallel to the substrate plane,or alternatively, in a direction perpendicular to the plane of thestationary base part (and thus, to the plane of the substrate). Thetransfer motion may also occur in both a lateral and a perpendiculardirection. The substrate can be adapted mountable on the movable part oron the stationary base part or, in some cases, even on both parts.

In an embodiment in which the movable part is designed such that can betransferred at right angles toward the stationary base part, thestationary part is equipped with a rabbet encircling its edge, while themovable part is provided with an outer edge that is shaped conforming tothe rabbet shape in order to provide a seal against the stationary basepart.

The construction according to the invention can be implemented in theform of some particularly advantageous embodiments. Accordingly, in thefirst preferred embodiment of the invention, the reactor chamber islocated in the manner disclosed in FI Patent No.97,730 in the interiorof a cold-wall vacuum reactor. In the present context, the term“cold-wall” reactor is used for making reference to an apparatus inwhich the reaction space and a possible reagent source adapted insidethe vacuum vessel are heated each separately. The heating system can beimplemented by enclosing each unit of the apparatus within a coverhaving one side facing the reactor (or source) equipped with a firsttemperature-equalizing jacket or plate and the other side facing thevacuum vessel wall possibly with another temperature-equalizing jacketor plate. Accordingly, there is adapted between said first and saidsecond jacket a heating means suitable for heating the surfaces of saidtemperature equalizing jackets. Such a heating means may comprisetubular resistive elements or planar heaters. The envelopes of thecovers enclosing the reaction space and the material sources are mostadvantageously made from a metal such as stainless steel, titanium oraluminum, or a suitable metal alloy or even a pure metal. By virtue ofsuch an insulation adapted between the reaction space and the reactantsources so as to keep them thermally insulated from each other and fromthe vacuum vessel, the internal temperature of the vacuum vessel can becontrolled independently from the temperatures of the sources and thereaction space. Resultingly, the vacuum vessel temperature can bemaintained essentially lower than that of the reaction space. As will beevident from the following description of an exemplifying embodiment, acold-wall reactor may also be constructed without any need to adapt thereagent sources to the interior of the vacuum vessel.

Depending on the type of reaction to be run, the cold-wall vacuum vesselmay be replaced by some other type of gastight vessel. This isparticularly true for a situation in which the reaction temperature isbelow about 200° C. In the context of the present invention, thestructure discussed herein is called generally with the name“pressure-tight vessel” or “vacuum vessel”, said term also includingso-called “vacuum vessels”. While the growth of a thin-film can becarried out under an atmospheric pressure or even a slight overpressure,operation under a partial vacuum will provide certain benefits inrespect to the purity of reactant and purge gases, among other things.

Most advantageously, the base part of the reactor chamber is supportedto the walls of the vacuum vessel so that the center axis of the supportmeans is coincident with the center point of the substrate. Thisarrangement prevents mechanical deformations of thermal origin fromaffecting the extremely critical positioning of the substrate when thereactor chamber is being closed. In order to minimize the effect ofthermally induced movements, it is also advantageous to pass thereactant infeed channels and the reaction product discharge channels via(a single and) the same wall of the pressure-tight vessel. When themovable part comprises a vertically movable plate transferred supportedby a transport rod, said rod is advantageously extended to the exteriorof the vacuum vessel through the same wall as is penetrated by the gaschannels. The transport rod can be made movable by manual oractuator-driven means.

The effect of thermal expansion forces on the positioning arrangementcan be eliminated with the help of flexible lines, for instance.

Furthermore, the base part may be made to comprise a portion of thevacuum vessel wall.

According to another preferred embodiment of the invention, thestationary base part of the reactor and/or its movable part is providedwith heating means, whereby their insulation can be accomplished asneeded by means of, e.g., reflective shields or active thermalinsulation elements. In respect to the details of such heating means,reflective shields and active thermal insulation elements, reference ismade to cited FI Patent No. 97,730. However, it is herein appropriate tomention that means generally called “active thermal insulation elements”typically include a cooling means and a reflective shield. Thus, it ispossible to establish between two hot spots a zone cooled to atemperature even lower than the cooler one of said hot spots. Typically,there are adapted thermal radiation reflecting envelopes about theheating means, whereby the number of the envelopes is at least one, butalso a greater number of envelopes may be used in concentricallyenclosing fashion. The sources are further surrounded by a coolingmeans, thus accomplishing the above-described cooled zone. This isnecessary in the case that the sources are adapted to the interior ofthe vacuum vessel. The cooling system may be based on a watercirculation, for instance. A similar cooling system may be constructedon the wall of the vacuum vessel, too.

The sealing of the movable part (that is, the sealing of the reactionchamber structure) can be accomplished by means of, e.g., an elastomerseal if possible within the constraints of the process and thetemperatures involved. Otherwise, the sealing must be made using, e.g.,a vacuum pump-out groove seal construction. The sealing effect of thevacuum pump-out groove may be augmented by feeding a protective gas intothe groove. This type of vacuum pump-out groove seal can be implementedby, e.g., machining on the surface of the planar structural elements ofthe chamber, close to the plate edges, a groove shaped as a loop passingclose to the edge contour of the plate, thus being capable of suckingaway any leaks. The vacuum pump-out groove is connected to a dischargechannel which is kept under a vacuum. The vacuum pump-out groove servesto prevent the access of contamination from the exterior of the reactorinto the reaction space or, conversely, the entering reactants fromescaping to the exterior side of the reaction space. The sealing flowpassing in the vacuum pump-out groove provides a maximally effectivefunction if the heaviest constriction to the flow of gases is arrangedin the vicinity of the trailing end of the substrate, close to thevacuum suction channel.

Advantageously, the edges of the opposedly mating parts of the reactionchamber are provided with notches cut at right angles to the plane ofthe plate so as to extend through the plate, thus forming the gaschannels of the reaction space when the reaction chamber structure isfully closed.

The reaction chamber can be shaped to accommodate a plurality ofsubstrates. Accordingly, the reaction chamber may be provided withsupport structures on which at least two substrates can be placed fordepositing thin-film structures on both substrates simultaneously.Generally, such support structures may comprise a rack having platformshelves or brackets provided thereon for supporting the substrates, oralternatively, a cassette unit consisting of stackable, preferablymutually identical, modular elements, whereby the interior spaces of atleast a part of the stackable elements are shaped so that they will beable to accommodate the substrates. Preferably these elements areprovided support brackets or platforms for the substrates. The supportstructures may be placed on the stationary base part of the reactor or,alternatively, on a movable base plate.

The racks may have either an open or a closed structure. In an openrack, the sides are left open so as to give a free access for thereactant gases from the infeed channel to the substrates and,conversely, from the substrates to the discharge channel. In a closedrack, the sides are partially closed, whereby the flow of the reactantgases takes place via infeed openings provided at one side of the rack,close to the substrates, while the other side is respectively equippedwith a discharge opening. Such a rack construction is provided withsubstrate holders comprised of shelf platforms or brackets serving tosupport the substrates. The latter arrangement also facilitatestwo-sided deposition on the substrates. If the substrate support isprovided by shelf platforms, they can be equipped with heating meansserving to stabilize the reaction temperature.

Instead of having a shelf construction, the reaction chamber may becomposed of essentially similar (even identical) modular elementsstacked on a base plate, whereby the proper design of the stack elementshave indents and grooves made thereto such that narrow passageways areformed acting as flow constrictors to the gas flowing through thereaction space. The elements have recesses made thereto with surfacesacting as substrate supports. The components of such a stackablecassette pack structure are fabricated from a material that is resistantto the reactants used in the ALE growth process. In this respect,advantageous materials are glass and the like silicate-basedcompositions, as well as various ceramics. The same groups of materials,as well as various metal alloys, are also suitable for making the racks.

When required, the internal surfaces of the process equipment may bepassivated by depositing thereon, e.g., an Al₂O₃ layer that can beformed using ALE from suitable starting materials such as aluminumchloride and water.

For multiple substrate processing applications, the inlet and outletlines (or feeding/pumping lines) of the reactant gases areadvantageously connected in a permanent manner to the reaction chamber.Further, the substrate rack or cassette structure is advantageouslyarranged in the reaction chamber so that the end of the gas dischargechannel meets the gas discharge opening of the rack or cassettestructure as tightly as possible. The substrates may also be arranged inan essentially upright position in the reaction space.

According to a third preferred embodiment of the invention, the movablepart is equipped with a substrate lift means, whose upright movementfacilitates easy placement of the substrate onto the robotic arm. Suchsubstrate lift means can comprise, e.g., pins inserted in holes made onthe substrate support surface so as to force the pegs to move at rightangles in respect to the support surface. The pins may be spring-loaded,whereby they automatically can lift the substrate upward off from thesubstrate support surface when the stacked pack is opened. The top endsof the pins are shaped downward tapering. Also the holes of the supportsurface are shaped similarly downward tapering, thus causing thedownward conical pin head to seal automatically against the hole seatwhen the substrate is placed in the reaction space. This arrangementserves to achieve a maximum tightness in order to prevent the reactantsfrom leaking freely into the reaction chamber. Otherwise, the reactantscontained in the reaction chamber could re-enter the reaction space atan incorrect time thereby interfering with action of the subsequentreactant pulses. Moreover, the reactants are often of a corrosive natureand their access to the loading chamber side would be detrimental to theprocess.

To overcome the sealing problems one starting material group can beintroduced into the reaction space via its top side and the other viaits bottom side. It must be noted herein that the movement of themovable part may also be arranged so as to occur in the principal plane,that is, in the lateral direction. Herein, the movable part (such as acover) is transferred by means of a pushing movement of the robotic arm.

Advantageously, also the gas inlet and outlet lines are sealed inrespect to the intermediary space. Typically, the load/transfer chamberis taken to a vacuum from about 10⁻⁶ bar to 10 mbar and, respectively,the ALE growth process is carried out at a pressure from about 0.1 mbarto 30 mbar. Thence, the ALE apparatus must be pumped to the lower one ofcited pressures for the load/unload steps. To speed up this phase, thereactor is advantageously equipped with a separate pump specificallydesigned for this purpose Such a pump can be of the turbomolecular pumpor cryopump variety, for instance.

As the film may also grow inadvertently on other surfaces than those ofthe substrate, stabilization of the reactor chamber temperaturecontributes to reduced formation of contaminating particles releasedfrom the flaking of films grown on the reactor surfaces. In the practiceof the art, the principal cause of flaking has been traced to thedifferences in the thermal expansion coefficients of the accumulatedfilm and the material of the reactor chamber walls.

ALE will give rise to thin films of even thickness. It is, howeverpossible further to reduce any variations in thickness of the thin-filmsby rotating the substrates about their central axis. The rotation can becontinuous or discontinuous. Continuous rotation can be implemented byrotating the substrate at a constant speed during the pulsing whilepreferably avoiding a rotational speed which would coincide with thepulsing frequency of the reactant feed which would lead to the pulsesalways entering the same sector of the substrates. Discontinuousrotation can be carried out by introducing a predetermined number ofsource chemical pulses then stopping pulsing and turning the substrateabout the central axis into a new position once the substrate is inmotionless state, and then continuing pulsing. The rotation angle can be1 to 359 degrees, preferably 90, 180 or 270 degrees. The substrate isrotated at least once during the deposition process. In bothembodiments, it is preferred to make the substrate holder rotatable.

Rotation of the substrate provides for an increase of the growth rate onsurface areas that otherwise should have a growth rate below the averagerate on the substrate and, conversely a decrease of the growth late onsurface areas that have a growth rate above the average. The rotationmethod is beneficial for growth processes where the growth rate of athin film is affected e.g. by the blocking of active surface sites withreaction by-products, such as HCl, HF and ammonium halides. Thisblocking is typical in the ALE process where acidic and basic componentsare present in the reaction chamber at the same time. In ALE of oxidesthe blocking is observed in the growth of Al₂O₃ from AlCl₃ and H₂O whereHCl is formed as a reaction by-product. HCl that is acid reacts furtherwith basic OH groups present on the surface of Al₂ O₃. In the reactionthe OH group is replaced with Cl that cannot be used as reactive site.H₂O is formed as a reaction by-product. Since the basic OH groups arealso the ones that are utilized in the metal compound reactions,non-uniform films are produced In ALE of nitrides the blocking isobserved in the TiN growth from TiCl₄ and NH₃ and in the growth W_(x)N_(y) from WF₆ and NH₃. On the TiN and W_(x) N_(y) surfaces, basicNH_(x) groups are present which can interact very strongly with HCl andHF that are formed in the reaction between NH_(x) groups and TiCl₄ andWF₆, respectively. In the metal nitride reactions especially thereaction of HCl or HF with the NH₂ groups leads to the replacement ofthe NH₂ group with Cl or F. NH₃ is formed as a by-product.

The specific properties and benefits of the invention will beappreciated from the following detailed description in which referenceis made to the appended drawings.

FIG. 1 is shown a top view of an apparatus according to the invention,wherein the vacuum vessel is denoted by reference numeral 1. Thereactant inlet lines are denoted by reference numeral 6. In the interiorof the vacuum vessel, there is adapted a reaction chamber 2, whosestationary base part (cf. later description) is mounted in the vacuumvessel on three supports. The circular contour of a substrate isoutlined in the diagram in a dashed line. The other dashed lines 17 inthe diagram outline the center axes of the support over the reactionspace. The reactant infeed and discharge lines 6, 4 and the reactionchamber 2 have a narrow cross section along their longitudinal axis andare elongated in order to achieve a “flattened” gas flow pattern and tominimize the size of the reaction space.

In FIGS. 2 and 3 is shown a more detailed structure of the apparatus. Asillustrated herein, the reaction chamber 2 adapted into the vacuumvessel 1 comprises a reaction space formed in a pack of superimposedlystacked planar elements 3. The set of planar elements 3 include a topplate 9 a base plate 10 and a heating plate 24 (cf. FIG. 3). Thin filmscan be grown in the reaction chamber onto a substrate using an ALEprocess. Reference numeral 4 denotes the connection of the reactionchamber to a line leading to the suction inlet of a vacuum pump, saidline being sealed against leaks from the exterior of the reactionchamber by means of a seal 5. Respectively, reference numeral 6 denotesan inlet line of vapor-phase reactants, said line being connected to thereactant inlet distribution plate 7.

In FIG. 3 is shown the technique of forming a looped vacuum pump-outgroove 8 for sucking away any possible leaks. The vacuum pump-out grooveis connected to a line 4 leading to the suction inlet of a vacuum pump.The function of the vacuum pump-out grooves is to prevent the access ofcontamination from the exterior of the reactor into the reaction spaceand, conversely, the reactants from escaping to the exterior side of thereaction space. Hence, it acts a kind of gastight seal for the reactor.

As noted above, the sealing of the planar elements against each othercan be implemented using elastomer seals. However, it is also possibleto attain a sufficient degree of tightness using straight planarsurfaces that act as the sealing seats between the planar elements.

The upper half of the reactor space forms the reactor chamber top plate9 and, respectively, the lower half is comprised of the base plate 10.The base plate is mounted on supports 11. Between the top and the baseplate, there is adapted a divider plate 12 serving to isolate thereactant flows from each other. To the bottom side of the base plate isattached a reactant inflow distribution plate 7 in which the gas flowsare divided into a linear front laterally over the reaction space. Thereactant infeed and discharge lines 6, 4 and the reaction chamber 2 havea narrow cross section along their longitudinal axis and are elongatedin order to achieve a “flattened” gas flow pattern and to minimize thesize of the reaction space.

In the embodiment illustrated in FIG. 3, the vapor-phase reactant pulsesof different reactant material groups are fed alternately into theinfeed line 6. Prior to the infeed, the reactant concentrations arehomogenized most appropriately by means of a protective gas flow in theinfeed line 6 or even earlier. In the infeed line, each vapor-phasereactant pulse having a planar, flattened flow pattern propagates with alinear, blunt front. The width of the flow pattern is equal to that ofthe substrate platform, which may be in the range of 5-50 cm, forinstance.

The flow propagating in the infeed manifold is distributed into ahomogeneous front by dimensioning the reaction space with itsconstrictions so that the conductance of the infeed channel 13 is muchlarger than the conductance through the capillary channels 14. In theinterior of the reaction chamber 2, the flow is homogenized by virtue ofa narrow suction slit 15 that acts as a gas flow constriction. Inpractical tests (performed with lean dosages), the gas flow front hasbeen found to be very straight. Uniform suction over the lateraldirection of the gas flow front is of primary importance, because gasmolecules tend to travel in the direction of the lowest pressure (causedby the highest suction power), whereby the linear contour of the gasflow front will become distorted if it is subjected to an unevensuction. On the other hand, laterally uniform suction can evenstraighten a gas flow front which is warped due to some reason.

In the embodiment of FIG. 3, the gas flow is passed via constrictionsthat are located both at a point 16 before the substrates as well as ata point 15 after the substrates. This arrangement can assure anextremely homogeneous flow over the substrates.

The center axis of the reaction chamber support means is adapted tocoincide with the center axis of the substrate 17. The support legs 11are provided with adjustment screws 29 by means of which the position ofthe reactor chamber can be set and locked in respect to the vacuumvessel. This construction aims to keep the center of the substratestationary and to direct the thermal expansion deformations radiallyoutward from said center. It is essential that the substrate position iskept stationary in order to permit the use of robotic handling means,whereby the robotic arm can place the substrate accurately into itssupport niche.

In FIG. 3 is shown an apparatus embodiment in which the reaction chamber2 can scalably admit a movable base plate 18 with its integral heatingplate 19. In this embodiment, the movable base of the reaction chamberacts as the substrate holder. The movable part has a beveled edgecontour that seals its edge against the reaction chamber edge. Thebeveled edge is provided with a groove or a number of grooves that canbe used as nitrogen purging grooves, seal grooves or vacuum pump-outgrooves. The edge of the base plate 18 may also have conical pins 30that guide the positioning of the substrate on the base plate 18.

The elements of the substrate lift means or pins 20 having a conicallower surface are sealed against the movable base plate by their conicalshape, thus permitting the reaction space to be isolated from theexterior of the reaction chamber. During a process run, the conical pins20 are withdrawn flush with the seat surface by means of springs 21. Theconical pins 20 are fastened to a support plate 22 that can betransferred mechanically or by an actuator means via a transport rod 23.

In order to provide homogeneous distribution of the inlet reactant gaspulses, it is possible to adapt the base plate 18 for rotation duringthe reaction. This means that, in practice, also the substrate liftmeans 20 are made rotatable. For example, a schematically shown motor 34may be provided for rotating the plates 18 by way of the rod 23.

The interior of the heating plate 19 and 24 can be equipped with avariety of different heater elements 25. The heating plates 19 and 24are pressed against the plates of the reaction chamber thus assuringefficient heat transfer. The heating of the reaction chamber isaugmented by locating reflective shields 26, 27, 28 about the stackedreaction space pack.

1. An apparatus for growing thin films onto a substrate by exposing thesubstrate to alternate surface reactions of vapor-phase reactants forforming a thin film onto the substrate by means of said surfacereactions, the apparatus comprising: a reaction chamber that includesinternal surfaces that define a reaction space into which said substratecan be transferred; inlet channels connected to said reaction chamberfor feeding therein said reactants used in said thin film growthprocess, said inlet channels being separated from each other untilopening into said reaction chamber; and outlet channels connected tosaid reaction chamber for discharging gaseous reaction products andexcess reactants, wherein said reaction chamber comprises at least twoparts, at least one part being movable with respect to the remainingpart(s) of the reaction chamber and adapted to be sealably closableagainst said remaining part of said reaction chamber; and furthercomprising controls configured to deliver at least two reactants throughthe inlet channels to the reaction space in alternate pulses.
 2. Theapparatus according to claim 1, wherein the parts of the reactionchamber are contained in a second chamber, both during processing andduring loading/unloading of a substrate.
 3. The apparatus according toclaim 2, wherein the second chamber comprises a vacuum vessel.
 4. Theapparatus according to claim 3, wherein the said reaction chambercomprises a base part mounted stationary in respect to the interior ofsaid vacuum vessel and a movable part adapted to be sealably closableagainst said base part of said reaction chamber.
 5. The apparatusaccording to claim 4, wherein said movable part is made transferableeither in a direction essentially parallel to the substrate plane, oralternatively, perpendicular to said plane.
 6. The apparatus accordingto claim 5, wherein said base part has a rabbet encircling its edge,while said movable part is provided with an outer edge that is shapedconforming to said rabbet shape in order to seal said movable partagainst said stationary base part.
 7. The apparatus according to claim4, wherein said base part of said reaction chamber acts as a substratesupport platform on which a substrate can be positioned.
 8. Theapparatus according to claim 4, wherein said reaction chamber is adaptedto the interior of a vacuum vessel so that said reaction chamber basepart is supported to the walls of said vacuum vessel or, alternatively,so that said base part forms a portion of the wall of said vacuumvessel.
 9. The apparatus according to claim 8, wherein said reactionchamber is supported to the walls of said vessel so that the center axisof the base part is coincident with the center point of the substrate inorder to prevent mechanical deformations of thermal origin fromaffecting the positioning of the substrate when the reactor chamber isbeing closed.
 10. The apparatus according to claim 4, wherein said basepart and said movable part of said reaction chamber are thermallyinsulated from said vacuum vessel by passive insulation, reflectiveshields and/or active heat insulation elements.
 11. The apparatusaccording to claim 4, wherein said movable part is sealed against saidbase part by means of metal and/or elastomer seals.
 12. The apparatusaccording to claim 4, wherein said movable part is sealed against saidbase part by means of a vacuum pump-out groove that may optionally beflushed with a protective gas.
 13. The apparatus according to claim 4,wherein said movable part is sealed against said base part by means ofplanar mating surfaces.
 14. The apparatus according to claim 4, whereinthe reactant inlet channels and the outlet channels of gaseous reactionproducts and excess reactants are passed via the same wall of the vacuumvessel.
 15. The apparatus according to claim 4, wherein the reactantinlet channels and the outlet channels of gaseous reaction products andexcess reactants are passed into the vacuum vessel via the top andbottom wall of the vacuum vessel, relative to the location of thereaction space housed therein.
 16. The apparatus according to claim 4,further comprising a support structure adaptable to the interior of saidreaction chamber, said support structure serving to hold at least twosubstrates for the deposition of thin films on said substrates in asimultaneous fashion.
 17. The apparatus according to claim 16, whereinsaid support structure comprises a rack having platform shelves orbrackets provided thereon for supporting said substrates, oralternatively, a cassette unit consisting of stackable modular elements.18. The apparatus according to claim 17, wherein said modular elementsare mutually identical.
 19. The apparatus according to claim 17, whereinat least some of said modular elements are designed to accommodate asubstrate.
 20. The apparatus according to claim 19, wherein saidsubstrate-accommodating elements exhibit support brackets or platformsfor the substrates.
 21. The apparatus according to claim 1, wherein saidmovable part is provided with substrate lift means, whose uprightmovement facilitates the elevation of said substrate off from saidsubstrate support platform and the subsequent placement of saidsubstrate onto a transfer means.
 22. The apparatus according to claim21, wherein said transfer means is a robotic arm.
 23. The apparatusaccording to claim 22, wherein said substrate lift means comprises pinsadapted on said substrate support platform so as to be movable at rightangles in respect to said support platform for elevating said substrateoff from the surface of said support platform.
 24. The apparatusaccording to claim 23, wherein said pins are spring-loaded and adaptedto move in holes made to the said substrate support platform
 25. Theapparatus according to claim 24, wherein the top ends of said pins areshaped downward tapering to seat against said substrate support platformand the seat holes of said substrate support surface are made similarlydownward tapering to the depth of said pin ends, thus allowing the pinsto be sealably lowered into said seat holes of said support platform.26. An apparatus for growing thin films onto a substrate by exposing thesubstrate to alternate surface reactions of vapor-phase reactants forforming a thin film onto the substrate, the apparatus comprising: areaction chamber positioned within the vacuum vessel and comprising afirst member and a second member, the second member being moveablebetween a first position in which the second member sealably engages thefirst member to define at least in part the reaction space and a secondposition in which the substrate can be loaded into or unloaded from thereaction space, inlet channels connected to said reaction chamber, saidinlet channels being separated from each other until opening into saidreaction chamber; outlet channels connected to said reaction chamber;and controls configured to deliver at least two reactants through theinlet channels to the reaction space in alternate pulses;
 27. Theapparatus of claim 26, further comprising a vacuum vessel defined by aplurality of walls that surround the second member such that the secondmember is in the vacuum vessel in the first and second positions. 28.The apparatus of claim 27, wherein the vacuum vessel and said reactionchamber are adapted such that the internal temperatures of the vacuumvessel and the reaction chamber can be independently controlled.
 29. Theapparatus of claim 28, further comprising an insulation elementpositioned between the reaction chamber and the walls of the vacuumvessel.
 30. The apparatus of claim 29 wherein the insulation elementcomprises a reflective shield.
 31. The apparatus of claim 30, whereinthe insulation element comprises an active cooling element.
 32. Theapparatus of claim 31, wherein the active cooling element comprises acoolant circulator.
 33. The apparatus of claim 28, further comprising anactive cooling system on the walls of the vacuum vessel.
 34. Theapparatus of claim 26, the second member forms a base plate configuredto hold the substrate during processing.
 35. The apparatus of claim 34,wherein the base plate is moveable in a direction parallel to asubstrate plane.
 36. The apparatus of claim 34, wherein the base plateis moveable in a direction perpendicular to a substrate plane.